Glass batch pellet production and drying process

ABSTRACT

In a rotary apparatus and a process for the production of layered glass batch pellets there is provided a drying process and a dryer for final drying of the pellets prior to their being fed to a melting furnace. Relatively cool moist pellets are fed into a static bed-type dryer to form a uniformly distributed pellet bed while a heating and drying gas is directed into the dryer in such a manner that the gas flows countercurrent to the direction of travel of the pellets through the dryer to maintain a temperature differential between the gas and the surface of the solid pellets in the bed throughout the bed&#39;s entire height in the range of approximately 5° F. to 120° F. and, ideally, within approximately 50° F.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part application of U.S.Application Ser. No. 123,153, filed Feb. 21, 1980, now U.S. Pat. No.4,293,324.

This invention relates generally to the production of pellets used inthe manufacture of glass and specifically to the process and apparatusutilized to dry pellets of agglomerated sand, limestone, and sodiumcarbonate or sodium hydroxide solutions utilized as preheated glassfurnace feed.

Glass such as soda-lime glass is produced by reacting and melting sand,soda ash, and limestone or dolomite and other glass batch ingredients ina furnace to form a homogeneous melt. The use of sodium carbonate in theglass batch introduces dust particles which are entrained in furnacegases. On contact with lining materials in the furnace, sodium carbonatedust particles accelerate the attack of the refractory materialsresulting in increased maintenance costs. One method known to suppressNa₂ CO₃ dust formation is to add water to the glass batch prior to itsbeing fed to the furnace. This water must be evaporated in the hightemperature atmosphere of the furnace and results in a curtailment offurnace melting capacity and an increase in fuel consumption per unit ofglass produced.

An improvement is obtained by the substitution of solutions of sodiumhydroxide for water and a portion of the sodium carbonate used. U.S.Pat. No. 3,149,983 issued Sept. 22, 1964, to L. Maris et al describesthe use of caustic soda with soda ash in the production of glass makingbatches containing sand. Glass batches produced by this method have atendency to cake and result in handling difficulties.

South African Patent Application No. 69-6971 by C. A. Sumner teaches thepreparation of agglomerated glass batch ingredients in a rotary drumhaving rods to develop a falling curtain of particles onto which acaustic soda solution is sprayed. Similarly, British Pat. No. 1,282,868issued July 26, 1972, to F. G. West-Oram teaches the production of aglass batch in pellet form from sand, limestone, and caustic soda in arotary dryer with flights. The pellets formed are heated to remove waterand to accelerate the reaction of the caustic soda with the sand.

Agglomerates prepared by the processes of South African Application No.69-6971 and British Pat. No. 1,282,868 as well as agglomerates producedin disk-type pelletizing apparatus are formed in sequentialpelletization and drying stages where caustic in the interior of thepellet is not completely carbonated. Such pellets are, therefore,hygroscopic and permit segregation of the soluble Na₂ O component duringdrying. These properties result in handling and storage problems andlead to non-homogeneous compositions of the molten glass.

The function of a drying step in the pelletization process is generallyto remove water supplied during the pelletizing process from theindividual pellets. The drying process generally strengthens the pelletsenabling them to withstand the rigors of handling and transport, as wellas providing a better melting characteristic. However, the drying ofpellets or the heating of solid agglomerates prior to feeding to amelting furnace, such as that utilized in glass making, presents anumber of difficulties. Exposing moist pellets to higher temperaturesduring an uncontrolled drying process can result in crumbling, smearing,or exploding of the pellets. This breakdown of the pellets may be due toexcessive strain induced by high thermal gradients within theagglomerated material or to excessive internal pressures such as occurswith super-heated steam where the rate of water volatilization exceedsthe rate of vapor diffusion to the internal pores of the individualpellets. Disintegration of the pellets normally occurs because the heattransfer rates to the interior of the pellets are in excess of the watervapor diffusion rates to the exterior. This normally results in theexplosion of the pellets.

An additional problem to maintaining pellet integrity occurs whenpellets are formed where crystalline sodium carbonate monohydrate (Na₂CO₃ ·H₂ O) serves as a bonding agent. Crystalline sodium carbonatemonohydrate has the apparent tendency to weaken as a bonding agent in aglass pellet batch at temperatures above a specific temperature rangebecause of the release of the water of hydration. This weakens themonohydrate bonding within the individual pellets. Above thispredetermined range the bond reforms as the pellet dries due to theevaporation of the water of hydration. At temperatures above thissensitive temperature range, anhydrous carbonate serves as a bondingagent to bond the pellets so that no substantial amount of pelletdegradation occurs. However, within this sensitive temperature range thepellets suffer substantial degradation if they are subjected tocontinuous or appreciable impingement against dryer flighting and eachother or to continuous mechanical movement during the final drying andpreheating process.

In contrast, the very slow heating and drying of pellets results in veryslow rates of water volatilization wherein the evaporation is normallyrestricted to the outer surface of the pellets. This allows the solublecomponents combined with the water to migrate to the surface of thepellet, thereby producing a heterogeneous composition from the surfaceinwardly. This latter is especially unacceptable for pellets used in themanufacture of glass since the more easily fusible soda-rich layer onthe outside flows away from the pellet during fusion in the glassfurnace and leaves a more refractory silica core which melts moreslowly. Such a process causes defects in the quality of glass producedbecause of the differences in density and viscosity.

The foregoing problems are solved in the design of the process andapparatus of the present invention by providing a static bed dryer thatminimizes pellet agitation and carefully controls pellet bed and dryingand heating gas temperatures differentials. The process and apparatus isutilizable with rotary pelletizing apparatus or any other appropriateapparatus which produces agglomerated glass batch ingredients inpelletized form for dehydration and preheating prior to the pelletsbeing fed to a melting furnace.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide in a process forproducing pelletized glass furnace feed from mixtures of sand,limestone, and solutions of sodium carbonate or sodium hydroxide astatic bed-type dryer which may be utilized to dehydrate and heat thepellets.

It is another object of the present invention to provide a process andapparatus in which relatively cool moist pellets are fed into a staticbed-type dryer to form a uniformly distributed pellet bed while aheating and drying gas is directed into the dryer in such a manner thatthe gas flows countercurrent to the direction of travel of the pelletsthrough the dryer to maintain a temperature differential between the gasand the surface of the solid pellets in the bed throughout the bed'sentire height in the range of approximately 5° F. to 120° F. and,ideally, within approximately 50° F.

It is a further object of the present invention to provide a gratemechanism or system for supporting the pellet bed in the static bed-typedryer that controls the transport rate of the bed of pellets from thetop of the dryer to the bottom uniformly across the entire cross-sectionof the bed.

It is another object of the present invention to employ feeddistribution apparatus at the top of the dryer to ensure uniformdistribution of the cool moist feed pellets over the top of the pelletbed.

It is a feature of the present invention that controlled heating ratesof the pellets in a static bed-type dryer is accomplished by gravitatingthe bed of pellets slowly from the top toward the bottom incounter-current contact with the heating and drying gas.

It is another feature of the present invention that the temperaturedifferential between the heating and drying gas and the surface of thesolid pellets is sufficiently restricted to limit the heat transfer rateto the solid pellets so that pellet explosions and degradation frominternal pressure buildup is avoided.

It is an advantage of the present invention that the static bed-typedryer is simple and inexpensive in operation.

It is another advantage of the present invention that the drying andheating of the pellets occurs within a sufficiently extended time periodto prevent thermal spalling or pellet degradation from internal pressurebuildup while the water vapor volatilization is sufficiently controlledto prevent internal pressure buildup and resultant explosion of thepellets.

These and other objects, features, and advantages are obtained in theprocess for the production of glass batch feed pellets by providing astatic bed-type dryer for the dehydration and drying of pellets thatensures uniform progression of the bed of pelletized agglomeratedmaterial down through the dryer and a uniform bed depth of the pelletsover the cross-section of the thermal treatment zone of the dryer, whilemaintaining a temperature differential between the surface of the solidpellets and the heating and drying gas of between 20° F. and 120° F. andideally 50° F.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of this invention will become apparent upon considerationof the following detailed disclosure of the invention, especially whenit is taken in conjunction with the accompanying drawings wherein:

FIG. 1 is an elevational view in section of the rotary apparatus whichmay be used in practicing the present invention;

FIG. 2 is a cross-sectional view taken along the lines 2--2 of FIG. 1;

FIG. 3 is an elevational view in section of an alternate embodiment ofthe rotary apparatus which may be used in practicing the presentinvention;

FIG. 4 and 5 are cross-sectional views taken, respectively, along lines4--4 and 5--5 of FIG. 3;

FIG. 6 is an elevational view in section of an additional alternateembodiment of the rotary apparatus which may be used in practicing thepresent invention;

FIG. 7 is a radial cross section taken along line 7--7 of FIG. 6;

FIG. 8 is a side-elevational view of the static bed dryer apparatusshowing in diagramatic detail an auger conveyor pellet distributionsystem;

FIG. 9 is a sectional view taken along the line 9--9 of FIG. 8;

FIG. 10 is an enlarged perspective view of the grate system; and

FIG. 11 is a side cross-sectional view of a gas distribution systemabove the grating for the static bed dryer shown in FIG. 9;

FIG. 12 is a bottom cross-sectional view, through lines 12--12 on FIG.11 of the gas distribution system;

FIG. 13 is a side cross-sectional view of the gas distribution systemthrough lines 13--13 of FIG. 12;

FIG. 14 is a side cross-sectional view of the gas distribution systemthrough lines 14--14 of FIG. 13;

FIG. 15 is a graphical representation of the temperature of the surfaceof the solid pellets as they progress through the dryer apparatus versusthe temperature of the drying and heating gas as it moves countercurrentto the pellets through the dryer, curves A showing the undesiredtemperature differential which occurs in prior art static bed-typedryers and curves B showing the temperature differential achieved by thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, and in particular FIG. 1, the rotaryapparatus of the present invention includes generally a hollowcylindrical shell 2 having an inner wall 3, a feed end 4, and adischarge end 62. Two riding rings 8 are mounted on the externalperipheral surface thereof. Shell 2 is mounted for rotation about itsaxis of elongation with the riding rings 8 riding on trunnion rollassemblies 11. Shell 2 is rotated by suitable ring gear and pinion drive12 by motor 13. The axis of rotation may be tilted slightly from thehorizontal with the discharge end 62 being the lower. Suitable endthrust rollers (not shown) may be provided, as well known in the art, tolimit axial movement of shell 2.

Feed end 4 of shell 2 is open. The aperture diameter of end plate 6 issmaller than the shell diameter to prevent spillage of the bed out ofthe feed end. A liquid inlet 22 and a dry feed chute 24 extend throughopening 4 into the interior of shell 2.

A stationary end housing 64 encloses the discharge end 62 of shell 2.End housing 64 is provided with gas duct 32.

Shell 2 contains a plurality of functional zones. The first zone is apelletizing zone 20 into which are fed solid feed materials through dryfeed chute 24 and liquid feeds through liquid inlet 22. Recycled feedparticles are introduced into pelletizing zone 20 through recycle spiralconveyor 42 surrounding the exterior surface of shell 2.

Adjacent to pelletizing zone 20 is drying zone 30. Drying zone 30contains at least one set of circumferentially spaced, radiallyextending flights 34. Radially extending flights 34 lift moist pelletsfrom the bed of drying zone 30 to the top of drying zone 30 and releasethe pellets to fall separately through the drying zone to the bed. Gasduct 32 introduces warm gases for drying the pellets. After flights 34,inner wall 3 of shell 2 is bare, forming recycle zone 40 which separatesdrying zone 30 from the classification zone 50.

Recycle zone 40 contains inlet 44 to spiral conveyor 42 which recycles aportion of the dried pellets to pelletizing zone 20. Another portion ofthe dried pellets is transported by elevator and deflector scoop 45 overdam ring 46, having adjustable gate 48, into classification zone 50.

Classification zone 50 is conically shaped with the smaller diameteradjacent to recycle zone 40. Dried pellets are fed from recycle zone 40by elevator and deflector scoop 45 to the center of classification zone50. Small pellets flow back towards recycle zone 40 and are readmittedto recycle zone 40 through adjustable gate 48 in dam ring 46. Largerpellets flow towards discharge end 62. Large pellets overflow end plate61 and enter discharge end 62 into end housing 64. From end housing 64,the dried pellets are discharged through opening 70.

As shown in FIG. 2, a set of flights 34 encircle the inner circumferenceof cylindrical shell 2 in drying zone 30. Rotation of cylindrical shell2 in a clockwise direction deposits dried pellets in inlet 44 of recyclespiral conveyor 42. Recycled pellets sliding inside recycle spiralconveyor 42 are returned to pelletizing zone 20. Elevator and deflectorscoop 45 deposits dried pellets into classification zone 50 downstreamfrom dam ring 46 having adjustable gate 48. Return of the dried pelletsfrom classification zone 50 to recycle zone 40 is controlled byadjustable gate 48.

In the alternate embodiment, shown in FIGS. 3, 4, and 5, the interiorsurfaces in the pelletizing, drying and recycle zones are madeaccessible to reciprocating scraper cage 52 to limit the uncontrolledbuildup of glass batch materials on these surfaces.

Reciprocating scraper cage 52 is comprised of a number of longitudinalbars or rods 54 positioned parallel to the drum axis and shorter thanthe combined length of pelletizing zone 20, drying zone 30, and recyclezone 40 by the length of the stroke for reciprocating scraper cage 52.Longitudinal bars 54 are formed into a rigid assembly by a series ofcircumferential rings 56 in pelletizing zone 20 and circumferentialrings 58 in drying zone 30. Circumferential rings 56 and 58 are spacedat intervals equal to the length of the stroke of scraper cage 52. Inpelletizing zone 20, the outside diameter of circumferential rings 56 isslightly less than the inside diameter of inner wall 3. As rings 56reciprocate along the surface of inner wall 3, the maximum thickness ofglass batch ingredients adhering to inner wall 3 is limited to the openclearance between circumferential rings 56 and inner wall 3. In dryingzone 30, the outside diameter of circumferential rings 58 is slightlyless than the inside diameter of the lips of radial extending flight 34to allow free movement of circumferential rings 58 back and forth alongthe flight lips. Attached to circumferential rings 58 in drying zone 30are blades 60 contoured to fit between adjacent radial extending flights34 and free to move back and forth between them. Buildup of glass batchmaterials on the sides of flights 34 and between flights 34 is therebydislodged by the action of blades 60 as reciprocating scraper cage 52moves back and forth inside drying zone 30 of shell 2. At suitableintervals around the perimeter of shell 2 are located reciprocatingpistons 74 driven hydraulically or pneumatically and connecting toreciprocating scraper cage 52 to provide the motive power for themovement of the reciprocating scraper cage 52. Since circumferentialrings 56 and 58 are spaced apart by the length of the stroke of cagemovement, the entire surface of inner wall 3 is thereby accessible toblades 60 attached to reciprocating scraper cage 52. If required, thecircumferential rings 56 and 58 can be offset or deformed to whateverextent required to avoid interference with other fixed obstructions inthe interior of shell 2 such as housings 76 required to supportreciprocating pistons 74 for scraper cage 52 as shown in FIGS. 3 and 5.Recycle zone 40 houses inlet 44 to spiral conveyor 42 which recycles aportion of the pellets entering from drying zone 30 to pelletizing zone20. Pellets passing over dam ring 46 enter supplemental drying zone 80fitted with lifter flights 82 which cascade the pellets through heateddrying gases fed through gas duct 32. Supplemental drying zone 80 isemployed to further reduce the moisture content of the pellets enteringfrom recycle zone 40 prior to their being fed to trommel screen 90. Thesupplemental drying of the pellets prevents the buildup of moist solidson trommel screen 90 which is employed as an alternate embodiment ofpellet classification. Undersized pellets and fines which pass throughthe openings of trommel screen 90 collect between dam ring 47 and endplate 61 and are passed through inlet 92 of spiral conveyor 42 to berecycled to pelletizing zone 20.

A further alternate embodiment of the apparatus of the present inventionis illustrated in FIGS. 6 and 7 in which a full length stationaryscraper blade 66 is positioned in the upper portion of cylindrical shell2. Stationary scraper blade 66 is attached to stationary axle 68 bysupport bars 72. Drying zone 30 houses cylindrical flight cage 84 havingan outside diameter slightly smaller than the inside diameter ofcylindrical shell 2 where stationary scraper blade 66 is positioned inthe intervening clearance between inner wall 3 of cylindrical shell 2and cylindrical flight cage 84. As best illustrated in FIG. 7,cylindrical flight cage 84 has end rings 86 to which are attached radialflights 88 and support spokes 94. Support spokes 94 are also attached tohub 96. Cylindrical flight cage 84 is in frictional contact with innerwall 3 along the bottom of cylindrical shell 2 whereby cylindricalflight cage 84 rotates in the direction in which cylindrical shell 2rotates as indicated in FIG. 7.

Referring briefly to FIGS. 1 and 3, the static bed dryer apparatus,indicated generally by the numeral 99 is seen diagrammatically connectedto the opening 70 in the end housing 62. It is to be understood thatother intermediate apparatus, such as suitable pellet storage bins andan appropriate pellet distribution system may be positioned between theopening 70 in end housing 62 and the final dryer 99. As shown generallyin FIGS. 8 and 9, the dryer 99 may have a suitable pellet distributionsystem connected to the opening 70 to receive the pellets and evenlydistribute them across the bed of the dryer 99.

One type of a suitable pellet distribution system is shown in FIGS. 8and 9 wherein rotating feeder 113 conveys pellets from opening 70 to aradially partitioned feed head 100 which is diagramatically illustrated.The radially partitioned feed head 100 equally distributes the pelletsthrough chutes 101 to a plurality of auger conveyors 102. Conveyors 102are comprised of auger flighting 103 and auger troughs 104; the augerflighting 103 distributing the pellets through the auger troughs 104 inan appropriate fashion to ensure uniform distribution of the pelletsacross the pellet bed that forms therebeneath in the dryer 99. Troughs104, for example, may be mounted for reciprocal linear movement or ofany other suitable design to permit the pellets to be directed inpredetermined quantities across the top of the pellet bed.

As can best be seen in FIGS. 8 and 9, dryer 99 is comprised of a framehaving pairs of opposing sides 105 and 106. As best seen in FIG. 9, awarm gas infeed port 107 feeds through one side of the sides 106 toforce warm gases up through the gratings, indicated generally by thenumeral 108. A gas exhaust port 109 may be provided near the top of thedryer 99 or, alternately, the gases may be permitted to rise out throughthe open top. Pellets are permitted to exit the dryer by passing throughthe hopper portion indicated generally by the numeral 110 whichterminates in a discharge port 111. The pellets then are discharged intoa suitable receiving vessel or may be deposited directly into the glassfurnace.

The gratings 108 have a handle portion 112 that extends from the side ofthe frame of the dryer 99 to permit the gratings to be moved laterally.The gratings 108 are seen in enlarged detail in FIG. 10.

The gratings 108 have three component parts all of which are locatedwithin what is the bottom receiving area for pellets which aredistributed to form a pellet bed across the gratings of the dryer by theauger conveyors 102 of the pellet distribution system. The firststationary lower grate portion 114 is seen as comprising individualT-shaped elements. Overlying the first stationary portion or lower grate114 is a second stationary portion or upper grate 115 which comprisesindividual inverted V-shaped elements. These inverted V-shaped elements115 are so shaped to facilitate the free flow of pellets downwardlythrough the gratings 108. Between the first stationary portion or lowergrate 114 and the second stationary portion or upper grate 115 is athird movable pellet transferring portion or shuttle grate 116. Theshuttle grate 116 occupies the vertical space between the upper grate115 and lower grate 114 and is fitted with uniformly spaced verticalpartitions 118 which block the pellet flow between open slots 120 in theupper grate 115 and open slots 119 in the lower grate 114. Deflectors117 guide pellets to open slots 120.

The height of the shuttle grate 116 need not coincide with the verticalclearance between the upper grate 115 and the lower grate 114. At aminimum however, the vertical height must extend sufficiently so thatthe space between the top of the vertical partitions 118 is at leastgreater than the pellet diameter to prevent crushing pellets which couldbecome caught between the bottom portion of upper stationary grate 115and the top of vertical partitions 118 of the shuttle grate 116. The topsurface 121 of the T-shaped lower grates 114 is flat and generallyhorizontal to allow for free lateral movement of the shuttle grate 116thereacross.

Pellets are thus fed downwardly from the auger conveyors 102 and theauger troughs 104, as indicated by the arrows in FIG. 8, to form a bedacross the gratings 108 of the dryer 99. The pellets initially willgravitate through the open slots 120 in the upper grate 115 and come torest on the flat top surface 121 of the lower grate 114. The lateralflow of pellets is blocked by the vertical partitions 118 of the shuttlegrate 116 thereby providing a confining and stabilizing structure topermit the bed to remain in a stable condition and build in heightacross the full width of the bed. At regulated time intervals, theshuttle grate 116 is then quickly shifted a lateral distance equal tothe spacing between any two adjacent vertical partitions 118. Thiscauses the pellets which were previously resting on the top surface 121of the lower stationary grate 114 to be moved over an open slot 119while additional pellet flow through the upper grate 114 and upper grateslots 120 is initiated to refill the vacant spaces in the shuttle grate116 between the now relocated vertical partitions 118. The rate ofdownward progression of the pellet bed is thus regulated by the shiftingof the shuttle grates 116 periodically at programmed time intervals oras desired.

The drying gas flow proceeds through the gas infeed port 107 and upthrough the gratings 108. The gas initially passes through the openslots 119 in the lower grate, through the distance between the upper andlower grates 114 and 115, respectively and then upwardly through theopen slots 120 of the upper grate 115. As the heating and drying gascomes into contact with the pellets, the gas passes through the openvoids between the stacked pellets. If desired, additional space for gastransmission may be provided by having small perforations or slots (notshown) in the inverted V-shaped elements of the upper grate 115.

FIG. 11 shows a preferred embodiment of the invention in a sidecross-sectional view of gas distribution system 122 above the gratings108 of static bed dryer 99 of FIG. 9. At selected positions in the uppergrate 115, the inverted V-shaped elements 115 are replaced by verticalhot gas ducts 123. Ducts 123 convey hot gas from the bottom of dryer 99upwards through pellet bed 125. Ducts 123 are also used to supportlateral extension ducts 124 which provide fixed channels for the lateralflow of the heating and drying gas substantially uniformly throughoutsolids bed 125 of dryer 99.

Upper level 126 of pellet bed 125 is at or near the top of vertical hotgas ducts 123, which are each capped with gas imprevious covers 127.Vertical hot gas ducts 123 are preferably constructed of a plurality offlanged box--like extension units 128. Lateral hot gas ducts 124 arepreferably inverted V-shaped elements which may be slotted, if desired.

Lateral damper units 130 are provided to work in cooperation withvertical hot gas ducts 123 to control the rate of flow of hot gas fromselected positions on the bottom of grating 108 to pellet bed 125.

FIG. 12 is bottom cross sectional view of a lateral damper unit 130through line 12--12 of FIG. 11. As shown in FIGS. 11 and 12, lateraldamper units 130 are comprised of a lower fixed plate 131 provided withopenings 132, which is secured to the sides of vertical hot gas ducts123 by welding 133 or otherwise. Upper movable plate 134 is alsoprovided with openings 135, and sliding of upper plate 134 permitsmatching openings 135 with openings 132 in lower plate 131 to permit alarge amount of hot gas to pass through. When it is desired to reducethe gas flow rate, as indicated by an undesirable rise in temperature ata given level in pellet bed 125, upper plate 134 is moved to reduce theportion of the matching openings in the two plates and thereby reducethe flow rate of hot gases in pellet bed 125 above the adjusted damperunit 130.

FIG. 13 is a side view of a lateral damper unit 130 through lines 13--13of FIG. 12 and FIG. 14 is a side view of a lateral damper unit 130through lines 14--14 of FIG. 13. These drawings also illustrate theoperation of the lateral damper unit 130 to control the flow rate of hotgas to upper level 126 of pellet bed 125.

Although lateral damper unit 130 is shown as a sliding bar, other typesof dampers, such as rotating perforated pipes and the like may beemployed.

The location and number of the vertical hot gas ducts 123 will vary withthe size of dryer 99. Generally a dryer having a width of about six feetwill utilize about two vertical hot gas ducts spaced equidistant fromdryer sides 106 and from each other. Lateral hot gas ducts 124 may beinverted V-angles which extend from dryer sides 106 through eachvertical hot gas duct 123, as shown in FIG. 14. The lateral hot gasducts 124 at each level of pellet bed 125 are substantially paralled toeach other, for ease of construction, but need not be. The number andsize of the lateral hot gas ducts per level will depend on the size ofdryer 99, but should not exceed the numbers and size which covers morethan about 25 percent of the cross sectional area of dryer 99. Thedistance between each level of lateral hot gas ducts 124 will depend onthe dryer size but generally is in the range from about 6 to 24 inches.The spacing between hot gas grates could be varied by appropriateselection of the height of the extension ducts and the number of hot gasducts employed across the width of the bed.

It should be noted that the upper grate 115 with its inverted V-shapedelements is off-set from the lower grate 114 with its T-shaped membersand their flat top surface portions 121. This vertical off-setting ofthe two grates facilitates the control of the flow of the pelletsdownwardly down through the pellet bed.

In the process of the present invention, glass batch pellets are formedin the pelletizing zone of the apparatus. The pellets are produced froma feed mix which includes ingredients which provide SiO₂, CaO, MgO, Na₂O, K₂ O, and other components which may be employed in the production ofglass.

Glass batch pellets produced by the process of the present invention maycontain the ingredients for producing commercial silicate glasses asgiven, for example, in Table 3 on pages 542-543 of volume 10 of theKirk-Othmer Encyclopedia of Chemical Technology, 2nd edition, 1966. Thistable includes compositions containing SiO₂, Al₂ O₃, B₂ O₃, Li₂ O, Na₂O, K₂ O, MgO, CaO, PbO as major constituents with other ingredientslisted as well, with the desired ratios employed in glass manufacturegiven in percent by weight. Preferred are glass batch pellets forsilicate glass compositions whose principle use is for flat glass,container glass, lighting ware, laboratory ware, light bulbs, andtumblers as well as in glass fiber insulation. More preferred glassbatch pellets are those whose compositions are suitable for theproduction of soda-lime glasses used in flat glass, containers andincandescent light bulbs or tubes. A widely employed soda-lime glasscomposition contains (as percent by weight) 72 percent silica, 15percent Na₂ O and 10-13 percent CaO (or CaO and MgO) with perhaps minoramounts of other metal oxides.

Sand is preferably used to supply the SiO₂ requirements although, forexample, alkali metal silicates may be employed. Any sand which issuitable for use in glass production may be employed. Sand particles arenormally employed having the size distribution ranges of the naturalproduct. Size reduction by crushing is generally not required.

The calcium carbonate source employed includes limestone, dolomite,calcium carbonate, aragonite, calcite and mixtures thereof. Mixtures ofa calcium carbonate source such as limestone or dolomite with lesseramounts of burnt lime, burnt dolomite, or hydrated lime may also beused. Suitably the calcium carbonate may have a particle sizecorresponding to that of the sand used. Particle sizes preferred, forexample, are those where about 90 percent of the particles are minus 100mesh and smaller. Pellet formation is facilitated by using more finelycomminuted-limestone or dolomite.

The Na₂ O requirements of the pellets are provided by employment of anaqueous solution of sodium hydroxide. Any suitable concentrations ofaqueous solutions of sodium hydroxide may be fed to pelletizing zone 20.While it is preferred to use NaOH to supply the entire Na₂ Orequirements of the glass batch mixture, as stronger pellets areobtained, it may be economically advantageous to use sodium carbonate,Na₂ CO₃, as a partial replacement for NaOH. Na₂ CO₃, dry or as anaqueous solution, can be employed to supply up to about 50 percent ofthe Na₂ O requirements. However, where mixtures of NaOH and Na₂ CO₃ areused, it is preferred that the substitution of Na₂ CO₃ for NaOH be inthe range of from about 1 to about 25 percent of the Na₂ O requirements.It is also preferred that the Na₂ CO₃ fed to the pelletizing zone be anaqueous solution.

In addition to these basic ingredients, the glass batch may contain alarge number of additives which are commonly used in glass productionsuch as feldspar and salt cake (Na₂ SO₄) as well as those which supplyFe₂ O₃, TiO₂, SO₂, and oxides of other metals of Groups III, IV, V, andVIII of the Periodic Table.

During starting operations, the moving bed may initially consist of anysuitable source of chemically compatible aggregated solids in theapproximate size range of the glass batch pellets to be produced. Forexample, silica pebbles or crushed limestone can be used initially toconstitute the moving bed. As suitably sized glass batch pellets areproduced, the aggregated solids in the bed are replaced.

In producing glass batch pellets by the process of the presentinvention, sand, the calcium carbonate source and other dry solidingredients are fed either separately or as a blend onto a rolling ortumbling bed of recycled pellets in the pelletizing zone in suitableamounts to provide the desired ratios of SiO₂ and CaO in the glassbatch.

The sodium hydroxide solution is fed or dispersed onto the rolling ortumbling bed of recycled pellets from the recycling zone. The feed rateof the aqueous solution is controlled to wet the solid ingredients andmaintain a cohesive condition between the sand, calcium carbonatesource, other glass batch ingredients, and recycle pellets whileproviding the desired amounts of Na₂ O. The dry glass forming solidingredients are preferably fed onto the bed in close proximity to thecaustic solution feed point. The sodium hydroxide solution is a sourceof soluble solids in the composition of the pellets. The sodiumhydroxide solution usually contains sufficient water to form a tackysurface on the recycle pellets to which the sand and limestone particlesadhere to form a new layer.

The recycle pellets used as seed particles in the pelletizing zoneprovide a core of sufficient strength so that the new layers of solidsformed by subsequent deposition of the feed ingredients can enduredrying without cracking or deformation. Water required to control thedegree of wetness of the bed is normally provided in the aqueoussolution of the Na₂ O source; however, water may also be addedseparately. The bed temperature may also be raised or lowered to controlthe rate of water evaporation.

Bed temperatures in the pelletization and drying zones are maintainedwithin predetermined limits covering those combinations of temperatureand NaOH caustic concentration for which the stable solid phase is themonohydrate of sodium carbonate, Na₂ CO₃.H₂ O ranging from temperaturesat or below approximately 110° C. at zero percent NaOH concentration toapproximately 18° C. at approximately 47 percent concentration NaOH. Assoon as sodium carbonate forms under conditions with temperaturesgreater than those at the concentrations in the monohydrate region justdescribed, the Na₂ CO₃ combines with a portion of the water containedwithin the pellet to form the crystalline monohydrate of sodiumcarbonate. The water of hydration of Na₂ CO₃.H₂ O is 17% of the weightof Na₂ CO₃. Pellets having a moisture content of more than 4% by weightcarry sufficient water to allow the hydration of all of the Na₂ CO₃ inthe pellet. Sodium carbonate is formed primarily by the reaction of theNaOH with the carbonate ion of the calcium carbonate source. Anadditional source of sodium carbonate of lesser importance is by theabsorption of CO₂ present in combustion gases fed to the pelletizingzone. Crystalline Na₂ CO₃.H₂ O is also formed when water is volatilizedfrom solutions of Na₂ CO₃. The formation of sodium carbonate monohydratein the pellet layer provides the desired bonding strength to the pellet.

The NaOH concentration of the residual solution in the pellets mayinitially be equal to that in the feed solution of NaOH. NaOHconcentration declines because of reaction with the carbonate ion in thecalcium carbonate source or with CO₂. Dilution of NaOH may also occurdue to supplemental additions of water. On the other hand, theevaporation of water within the drying zone tends to increase theconcentration of the NaOH. The temperature of the combustion gases inthe drying zone may be increased or decreased to establish control overthe bed temperature and the water evaporation rate.

When 50 percent NaOH is used as the feed concentration of the NaOH, ithas been determined that the rate of reaction of the NaOH with dolomiteas the calcium carbonate source is sufficiently rapid to reduce theresidual NaOH concentration in the liquid phase present in the pelletsto concentrations less than 30 percent. In this instance, to formcrystalline Na₂ CO₃.H₂ O, the bed temperature can be allowed to reach amaximum temperature of about 60° C. On the other hand, where calcite oraragonite are used as calcium carbonate sources, the reaction was foundto be somewhat slower, allowing residual concentrations of NaOH in theliquid phase in the pellets to remain near 35 percent. Under theseconditions, the maximum allowable bed temperature was about 50° C. Ifbed temperatures exceed these maximum limits, the crystalline Na₂ CO₃.H₂O melts releasing the water of hydration. Bond strength provided by thecrystalline monohydrate is then reduced and the pellet structure isdegraded.

In further consideration of the allowable combinations of bedtemperature and NaOH concentration, it has been shown that use of moredilute NaOH feed solutions allows an increase in the operating rate ofthe pelletizer. Thus less reaction time is needed to reduce the causticconcentration to sufficiently low values to permit bed temperatures inthe range from about 50° to about 80° C., to be employed, bedtemperatures which are preferred during pellet formation. Thesepreferred bed temperatures are conducive to faster water evaporationrates required because of the use of the more dilute NaOH. The higherallowable bed temperature for dolomite as compared to calcite oraragonite as calcium carbonate sources may be attributed to the morerapid reaction of NaOH with the MgCO₃ in the dolomite. By similaranalogies, it may also be inferred that more finely pulverized feeds oflimestone or dolomite will react with the NaOH more rapidly than coarsermaterials, thereby allowing operation at both a higher bed temperatureand also at a higher operating rate. Operation of the pelletizing zoneat bed temperatures less than 50° C. is permissible with the appropriateconcentrations of NaOH solutions. However, the reaction rate of NaOHwith the calcium carbonate source and the water evaporation rate areboth retarded and the maximum operating rate of the pelletizer isreduced.

The utilization of the relatively dilute sources of NaOH solution in therange of 10 to 30 percent allows the use of bed temperatures in therange of 60° to 100° C., suitably high to achieve an accelerated rate ofreaction of the NaOH with the CaCO₃ or MgCO₃ and also provide for anequally rapid rate of volatilization of the water fed to the pelletizerwith the caustic solution.

Bed transport within the rotating drum moves the moist coated seedpellets into the drying zone. Heated gas, for example, air or combustiongases, contacts the glass batch pellets to evaporate and remove water.Water in the caustic solution in excess of that required to maintainpellet residual moisture content is immediately volatilized in thedrying zone. This causes the Na₂ CO₃ formed by the reaction of NaOH withcarbonate ion or CO₂ to crystallize and prevents the migration of thesoluble Na₂ O components into subsequently deposited layers. Radiallifter flights installed in the drying zone lift the layered pellets tothe upper portion of the drying zone and release them to fall separatelythrough the heated gas and thus provide for controlled heat transfer.While radial flights are preferred, the drying zone may comprise arotary kiln or contain rotary louvers.

Where the heated gas used for drying is a combustion gas, carbon dioxideis present. During the evaporation of water, absorption of carbondioxide from the drying gases onto the newly deposited layer reacts withthe NaOH and aids in the formation of crystalline bonds of sodiumcarbonate. This further prevents the migration of soluble soda compoundsinto subsequently deposited layers of feed materials onto the surface ofthe pellets. Carbon dioxide also reacts with calcium hydroxide [Ca(OH)₂] produced during the reaction of NaOH with the calcium carbonatesource, and where present, with magnesium hydroxide to form calciumcarbonate and magnesium carbonate. Formation of crystalline bonds in thenewly deposited layers strengthens and toughens the pellets. Theformation of Na₂ CO₃ by the reaction of NaOH with CaCO₃ and MgCO₃ or bythe absorption of CO₂ by NaOH also neutralizes hygroscopic properties ofthe caustic soda.

Gases, such as air and flue gases used in drying the pellets are attemperatures in the range of from about 100° to about 300° C., andpreferably from about 100° to about 200° C. Where combustion gasescontaining CO₂ are used in the drying zone, suitable amounts of CO₂include those from about 1 to about 30 percent by weight of the hot gas.

The pellets are retained in the drying zone for a period sufficientlylong to evaporate water in excess of that required to provide the driedpellets with the desired residual moisture content. Suitable dryingtimes include those of from about 2 to about 20 minutes. The driedpellets have a residual moisture content in the range of from about 4 toabout 12 percent, preferably from about 5 to about 9 percent and morepreferably from about 6 to about 8 by weight. Residual moisture includeswater of hydration and free water present in the pellet.

Dried pellets pass through the inlet of spiral recycle conveyor 42 andare returned to the pelletizing zone as recycle pellets. Also returnedto the pelletizing zone through spiral recycle conveyor 42 areunagglomerated dry solids such as sand and limestone. A portion of driedlayered pellets is transferred to classification zone 50 by elevator anddeflector scoop 45. Scoop 45 is adjustable between a zero bed depthsetting wherein the inlet end of the scoop 45 is in engagement with theinternal periphery of shell 2 and a full bed depth wherein the inlet endof scoop 45 is at a height at least equal to that of adjustable gate 48.

Pellets from recycle zone 40 are deposited by elevator and deflectorscoop 45 near the center of classification zone 50. In the rotatingconical classification zone, the smaller pellets segregate at thesmaller diameter adjacent to recycle zone 40. The flow of smallerpellets back into recycle zone 40 is regulated by adjustable gate 48. Tominimize direct by-passing of small pellets and unagglomerated sand andlimestone back into the classification zone, elevator and deflectorscoop 45 is positioned in relation to adjustable gate 48 and conveyorinlet 44. Larger pellets deposited in classification zone 50 will movetowards the larger diameter of the cone. The bed depth in theclassification zone is regulated by end plate 61. Pellets overflowingthis barrier enter discharge end 62 and are discharged from end housing64 through opening 70.

Classified layered pellets exiting from classification zone 50 have adiameter of from about 1.5 to about 26 and preferably from about 3 toabout 20 millimeters.

During extended periods of operation, a slow progressive rate of buildupof glass batch materials occurs on inner wall 3 within pelletizing zone20 and drying zone 30 where contact occurs with the pelletized bed ofmaterial. This buildup is undesirable as it will ultimately reduce theoperating efficiency in the pelletizing and drying zones. The buildup ofglass batch materials can be limited to inconsequential amounts by useof suitably designed scraper systems. One such system is shown anddescribed in FIG. 3. A slow rate of reciprocation of such areciprocating scraper cage dislodges any buildup in excess of theclearance between the moving scraper surface and the surfaces of innerwall 3. The dislodged material is reincorporated in or recycled to thebed of pellets in the pelletizing zone. The freshly deposited solidsforming the buildup are of a sufficiently soft texture to allow easydisengagement without reaction stress in excess of the rigidity limitsof the cage assembly. In an appropriately designed cage, the unscrapedinner surfaces of the scraper cage itself are of an insufficient area tocause concern for glass batch materials adhering to these surfaces.

In an alternate embodiment of the scraper blade illustrated in FIGS. 6and 7, the buildup of solids on the walls of the apparatus is limited bya stationary scraper blade positioned close to the inner wall of thedrum. Clearance for the operation of a stationary scraper blade in thedrying zone is achieved by using a detached flight cage in lieu of therigidly attached flights employed in the apparatus illustrated in FIGS.1, 2 and 3.

In the alternate embodiment for classification of the product pelletsusing a trommel screen as shown in FIG. 3, the pellets from recycle zone40 are dried further in supplementary drying zone 80 sufficiently toprevent any significant buildup of moist solids on the trommel screenwires. The buildup of solids impairs the classification of pellets bythe trommel screen. Superficial drying is provided by a supplementarydrying section furnished with radial flights to induce accelerated heattransfer between the hot gases and the pellets by cascade of the pelletsthrough the hot gases. These dried pellets, having a moisture content offrom about 0.1 to about 1.0 percent by weight less than pellets enteringsupplementary drying zone 80, then progress onto the trommel screenwhere "on-size" pellets continue to the discharge opening of thepelletizer drum. The undersized pellets and fines fall through thescreen and are returned to pelletizing zone 20 through spiral conveyor92 and 42.

The novel process of the present invention produces multi-layeredpellets having a homogeneous cross-sectional composition from theinterior to the surface. By producing pellets by the formation of thinlayers (onion-skin increments) by depositing moist glass batchingredients on a dry substrate on each pass through the recycle system,migration of soluble NaOH is prevented. The repeated recycle of thepellet through the pelletization zone and drying zone, for example, upto as many as 20 recycles, converts the major portion of NaOH in thelayer to the less soluble and non-hygroscopic sodium carbonatemonohydrate. This process imparts strength and hardness to themultilayer pellet which is not attainable by the single-step procedurespreviously employed in preparing glass batch pellets. Because of the lowconcentrations of residual moisture in pellets recycled or recoveredfrom the drying zone, dehydration and pre-heating can then be effectedin a subsequent operation with no further migration of soluble sodacompounds occurring. Multilayered pellets produced by the novel processof the present invention can endure storage and handling and pre-heatingtreatments without excessive breakdown and dust formation.

The layered pellets for glass furnace feed in the apparatus and processof FIGS. 1-7 generally contain from about 4-8 percent by weight of waterprior to the final drying step in dryer 99. However, pellets for glassfurnace feed can be prepared by other techniques, such as rotating pansor discs, rotary drums and the like. Pellets prepared by the lattertechniques generally contain considerably more moisture, for examplefrom about 8 to about 20 percent by weight of water. The novel staticbed dryer 99 of FIGS. 11-14 can be utilized to dry pellets for glassfurnace feed prepared by a variety of techniques, including thosedescribed above, where the water content may range from about 4 to about20 percent by weight.

In carrying out the process of this invention in the apparatus of thepreferred embodiment shown in FIGS. 11-14, pellets are fed to the top ofdryer 99 to fill the pellet bed 125 to the upper level 126, with shuttlegrate 116 in a closed position, as shown in FIG. 11. Vertical partitions118 match the top surfaces 121 of lower grate 114 to retain the pelletswithin dryer 99, as illustrated on the right side of FIG. 11. Hot gas isforced up through open slots 119 in lower grate 114 through open slots120 in upper grate 115, and up through the pellet bed 125. Wherevertical hot gas ducts 123 are positioned above open slot 120, a portionof the hot gas passes out ports 137 into the pellet bed and up to theupper level 126. The remainder of the hot gas entering the bottom ofeach vertical hot gas duct 123 rises towards cover 127, and as it passeseach lateral hot gas duct 124, a portion of the hot gas is distributedlaterally to pellet bed across ducts 124. The hot gas entering underupper grates 115 is at a relatively low temperature after it passesthrough pellet bed 125 and it reaches upper level 126, as it is inconventional drying techniques. However, in dryer 99, the hot gasentering the bed at any level of lateral hot gas ducts 124 is stillsubstantially at the elevated temperatures of the feed hot gas.

As a result the gases entering the pellet bed 125 through grate members114 and 115 which have been cooled by counter current heat transfer tolower temperature pellets will be re-heated in proportion to the hot gasflow through ducts 124 relative to the cooler gas flowrate in the pelletbed 125. An essentially uniform temperature differential between the gasand the pellets is thereby maintained.

At start-up of the novel dryer of this invention, an initial period ofbatch operation of the dryer is programmed to dehydrate and preheat thebed as a batch at a suitably slow rate. This batch operation is achievedby omitting bed discharge from the dryer until the pellets in thelowermost zone of the bed of pellets have been adequately dried andpreheated. Hot gas flow during the initial batch operation is typicallyset at 20 to 60% of the normal gas flow rate for continuous operation.The temperature of the hot gas feed may also be reduced for initialstartup operation. This low gas flow rate permits the heat transfer rateto the cool moist pellets to be limited to sufficiently low values toprevent explosion of the pellets during dehydration and avoid spallingor pellet disintegration during the subsequent pre-heating period of thedehydrated pellets.

After the lowermost pellets have been dehydrated and preheated at thereduced gas rate, the continuous normal operation of the dryer cancommence. The grating 108 is operated to discharge preheated pellets ata programmed rate while the feed distribution mechanism utilizing theaugers 102 is activated to maintain the top pellet level at a normaloperating level. Concurrently the hot gas flow rate is increased toestablish the desired heating level at the bottom of the pellet bed.

Additional heated gas is admitted into the pellet bed through gasdistribution system 122 of FIG. 11. The vertical hot gas ducts 123 andlateral hot gas ducts 124 at higher levels in the pellet bed maintainthe desired differential temperatures between the gas and pellets withina specified control range typically ranging from a minimum value ofabout 5° F. to a maximum differential not exceeding 120° F. andgenerally not more than 100° F. The temperature differential must besufficient to induce sufficient heat transfer over the dehydrating zoneof the pellet bed to volatize the water in the pelletized feed. Lateraldampers 130 are adjusted to give the damper settings which maintain thisdesired temperature differential, in response to the amount of waterthat must be volatized from the pellets within the dryer.

The effect of distributing hot gas in the novel dryer of this inventionis shown in FIG. 15 which shows the plot of temperature with time forpellets (Curve A-2) contacted with hot gases in a conventionalcounter-current static dryer where pellets are fed at the top andheating gas is fed solely at the bottom. Curve A-1 shows the temperatureof the heating gas corresponding to the pellet temperature. In addition,FIG. 15 also shows the pellet (Curve B-2) and gas temperatures (CurveB-1) for drying pellets in the novel static dryer of this invention.

Curve A-2 shows an initial period from 5 to 20 minutes of inactivity inthe temperature of the pellets after the pellets are introduced into thedryer because no additional available heat remains in the gases withinthe pellet bed. From the time period 20 to 40 minutes the available heatin the hot gases within the pellet bed increases rapidly due to theincreasing temperature difference between the hot gases and the coolmoist pellets. Heat transfer is thereby accelerated to allow pellets toexplode due to formulation of superheated steam within the pellets. Therapid rate of pellet temperature continues after dehydration iscompleted from 40 to 50 minutes because of countercurrent contact withhot feed gas entering below the grate until the temperature differentialbetween the pellets and the gases again becomes negligible therebyinducing further inactivity in heat transfer over the time period 50 to80 minutes. Pellets may spall or crack during the rapid rate oftemperature changes from 40 to 50 minutes because of strain due toexcess thermal gradients.

Curve A-2 shows an initial priod from about 5 to 20 minutes ofinactivity in the temperature of the pellets after the pellets areintroduced into the dryer because no additional available heat remainsin the gases within the pellet bed. For materials not susceptible todisintegration by thermal strain, this increased rate of temperaturechange is of no consequence, but in pellets of the type described hereinwhich are highly susceptible to disintegration due to excessive thermalstrain, this rapid temperature differential will increase to a levelthat permits the dangerous buildup of internal pressure within thepellets, which usually results in explosions and thermal spalling. TheseA plots are based upon the use of approximately one inch diametergenerally spherical pellets having a volumetric heat transfercoefficient of 0.4 BTU/Ft² -°F.-MIN. and a heat capacity of 30 BTU/FT³-°F.

With further reference to FIG. 15, curve B-2 shows the temperature ofthe pellets throughout pellet bed 125 over a period of about 80 minutes,with the corresponding heated gas temperature (B-1) for a static dryer99 of the present invention. There are four levels of lateral hot gasducts 124 which distribute hot gases to the bed at levels when thepellets have been in the bed at periods of about 20, 30, 40, and 50minutes. At these points, the temperature of heating gas increasesappreciably sharply, as indicated at points 1,2,3, and 4 on curve B-1,and the temperature then decreases as the heat from the gas isdistributed to the pellet bed 125.

In the novel dryer of this invention, where close control of thetemperature differential is effected, inactive bed phases are avoided sothat from approximately 5 to 50 minutes the bed is experiencinggenerally uniform dehydration and from approximately 50+ to 80 minutesthe bed is experiencing generally uniform heating. The exponentiallydeclining value of the temperature differential between the gas and thesolid pellets is incrementally expanded by the injection of additionalhot gas into the pellet bed at these preselected bed heights andpreselected bed locations across the cross-section of the pellet bed.The effect of controlling the temperature differential between thesurface of the solid pellets and the countercurrent heating and dryinggas is evident when the temperature profiles for the gases and thepellets are compared in the hydration zones for plots A and B.

It is apparent that with higher water content of the individual pellets,a higher temperature differential between the gas and the pellets in thedehydration zone of the pellet bed must be provided to accomplish thevolatization of the water during the desired time interval allocated todehydrate the pellets in the dryer. With a drying apparatus of thedesign disclosed herein pellets with only 5% by weight of water contentcan normally be dehydrated in approximately 40 minutes with atemperature differential of about 20° F. whereas pellets with a 15% byweight water content have typically required a temperature differentialof approximately 60° F. to effect volatilization of the water during thesame time period. The optimum temperature differential appears to beapproximately 50° F., but may range from about 5° to about 120° F.

It should also be noted that varying the diameter of the pellets to bedried and preheated will affect the time and the temperature required toeffect the volatilization of water. If the dryer is operated with thesame temperature differential between the drying gas and the pellets,the total retention time that the pellets spend within the dryer can bereduced in proportion to the pellets size. A typical pellet bedretention time for pellets of approximately 1 inch diameter is 120minutes.

It should also be noted that with a smaller dryer bed cross-section, theheight of the bed of pellets (measured from the pellets resting on thetop flat surface 121 of the lower grate 114 to the very top of thepellets beneath the augers 102) must be increased to meet retention timerequirements. Bed cross-section and water content by weight of theindividual pellets are factors which influence the drying gas velocitythrough the bed.

A change in the cross-sectional area of the pellet bed will produce achange in the pressure drop that occurs within the pellet bed from topto bottom. Pressure drop through the bed of pellets increases inverselyas the square of the pellet bed cross-sectional area changes since thedrying gas must flow at higher velocity through a smallercross-sectional bed that correspondingly is also deeper to maintain afixed or relatively constant pellet bed retention time.

Novel layered spherical pellets produced by the process of the presentinvention have a controlled range of particle sizes. The pellets are ofa homogeneous composition in which segregation of components such as Na₂O is minimized. The pellets are non-cohesive so that each pellet canfloat independently on molten glass. The novel layered spherical pelletshave a specific gravity in the range of from about 1.90 to about 2.30,and preferably from about 2.00 to about 2.20. These high densitiesresult in the layered pellets having good thermal conductivity. However,the density of the pellets is less than that of the glass melt so thatthe pellets melt on the surface of molten glass without sinking into themolten glass thereby optimizing heat transfer in the glass productionprocess. The layered pellets melt at temperatures below whichsubstantial amounts of pollutants such as nitrogen oxides are generatedfrom the glass melt. The novel layered spherical pellets arenon-hygroscopic and can be stored for extended periods of time withoutclumping or agglomerating. Crushing strengths are in the range of fromabout 50 to greater than 250 pounds and provide the pellets withsufficient hardness so that additional handling will not producesignificant amounts of dust or fine particles. In addition, the pelletsare resilient and can be dropped onto hard surfaces without splitting orshattering.

The following examples are intended to further illustrate the process ofthe present invention and are offered without any intent to pose anylimitations upon the present invention.

EXAMPLE 1

A drum, cylindrical in shape, 30.5 centimeters in diameter and 25.4centimeters long and having a sealed flat bottom was mounted on a flangeso that the drum axis was horizontal. The flange was attached to thehorizontal output shaft of a motorized speed reducer geared for anoutput shaft speed of 29 rpm. The other end of the drum was fitted witha conical restriction terminating in an access opening 15.2 centimetersin diameter. The interior of the drum was fitted with 12 radial flights2.5 centimeters in height secured at points approximately equidistantaround the interior wall. As the initial seed bed, about 4.5 kilogramsof screen pellets about 6.5 millimeters in diameter were loaded into thedrum, a mixture of screened sand (+0.42 mm) and finely pulverizedlimestone was prepared in amounts which provided a SiO₂ ratio to CaO of72:13. The mixture was added to the drum in 1.125 kilogram increments.Heat was provided by a gas-oxygen torch burning liquified petroleum gas(LPG) whose flame was projected into the interior of the drum throughthe upper portion of the 15.2 cms. access opening. A 50 percent causticsolution was projected horizontally into the drum in the lower portionof the access opening and at an angle with respect to the drum axis toallow caustic impingement near the mid-point of the cascading bed insidethe drum. Caustic projection was by feeding the caustic at a regulatedrate into an air stream of sufficient velocity to break the liquid intodroplets and to project these droplets about 20.3 cms. into the interiorof the drum before contacting the cascading dry solids and recyclepellets. The caustic solution was fed to the drum in amounts whichprovided a ratio of SiO₂ to Na₂ O of 72:15 to the glass batch feed. Thebed temperature was in the range of 90° to 100° C. The pelletizationprocess was run with the drum rotating at 29 rpm until a total of 9kilograms of the sand-limestone feed mix had been fed to the drum. Thecentral portion of the access opening between the flame and the causticspray provided access for intermittent manual additions of dry feed.Excess material inside the drum, due to buildup of the bed, was allowedto spill out of the access opening into a pan. The material was screenedand undersized material returned to the drum through the access opening.CO₂ present in the combustion gas contributed to the carbonation of thecaustic in the feed. The time required to pelletize 9 kilograms of feedmix was about 2 hours. At the end of the pelletization run, 6.75 kgs. ofpellets were recovered along with 2.25 kgs. of unpelletized fines. Thelayered pellets were dried. The dry layered pellets produced weresufficiently hard so that they could not be crushed by hand. Pelletsizes in the product were in the range of from 3 to 16 millimeters indiameter. Due to the short length of the drum, the pelletization anddrying zones in this example were actually super-imposed onto oneanother. Recycle was therefore not required. While this tended to impairthe degree of controllability of bed moisture for maximum pelletstrength, the pelletized product exhibited sufficient strength forsubsequent pre-heating without breaking.

EXAMPLE 2

A dry blend of glass batch ingredients was prepared having the followingcomposition:

    ______________________________________                                        Component:                                                                             Sand    Dolomite Feldspar                                                                             Salt Cake                                                                             Total                                ______________________________________                                        % by weight:                                                                           70.6    21.2     7.1    1.1     100.0                                ______________________________________                                    

Pelletizing apparatus of the type illustrated in FIG. 6 was employedhaving a pelletizing zone, a drying zone and a conical classificationzone. The apparatus was 45.7 cms. in diameter and 152.4 cms. long. Astationary scraper blade supported on a tubular axle coincidingapproximately with the axis of the apparatus was provided to limit thebuildup of glass batch materials on the inner wall of the pelletizingzone and drying zone. A detached flight cage, also supported by thetubular axle, was employed having a diameter slightly smaller than theinternal diameter of the pelletizing apparatus to provide clearance forthe scraper blade. The flight cage rested on the bottom side of thepelletizer and rotated with the pelletizer to induce cascading of thepellets which remained enmeshed with the cage. Attached to the dischargeend of the pelletizer was a burner for gaseous fuels to provide the hotcombustion gases necessary for heating the bed of pelletized solids andfor volatilizing water.

The pelletizer was loaded with a 13.5 kilogram bed of pellets 3 to 8millimeters in diameter. The dry blend was fed continuously onto therevolving bed in the feed end of the pelletizer. Also sprayedcontinuously onto the revolving bed was a 50 percent aqueous solution ofNaOH at a rate of 0.9 kgs. per 2.54 kgs. of dry blend. An axialinclination of the pelletizer of 0.23 cms. per meter transported thepellets progressively from the feed end to the discharge end of thepelletizer. A major portion of the pellets reaching the recycle zoneentered the spiral conveyor and were recycled to the pelletizing zone.The remaining pellets overflowed into the conical classification zoneand were collected as product at the discharge end. The pellet bedtemperature was maintained in the range of 45° to 55° C. Water fed inassociation with the 50 percent NaOH solution was volatilized at ratesin the range of 3.2 to 6.75 kilograms per hour. Dry blend feed ratesallowed by these water evaporation rates ranged from 32 to 45 kilogramsper hour. Under conditions of excessive drying of the pellet bed,supplemental additions of water were made with the caustic solution tomaintain pellet moisture levels in the range of 6 to 8 percent.

The size of the spherical pellets produced ranged from 8 to 15millimeters in diameter. The pellets were too hard to be crushed ordeformed by finger pressure. Pellets dropped onto hard surfaces fromheights of 20.3 to 25.4 cms. remained intact and exhibited resiliency.

EXAMPLE 3

A dry blend of glass batch ingredients was prepared having the followingcomposition:

    ______________________________________                                        Component:                                                                             Sand   Aragonite Feldspar                                                                             Salt Cake                                                                             Total                                ______________________________________                                        % by weight:                                                                           70.6   21.2      7.1    1.1     100.00                               ______________________________________                                    

Aragonite is a mineral form of calcium carbonate. Using the apparatusand procedure of Example 2, a 50 percent NaOH solution and the dry blendwere continuously fed into the pelletizer in amounts of 0.9 kg. of NaOHper 2.54 kgs. of dry blend. Pellet bed operating temperatures in therange of 40° to 45° C. were found to be most conducive towards thegrowth of pellets with satisfactory strength. At these temperatures, thevolatilization rate of water was about 5 lbs./hr. Dry blend feed ratesemployed at this water evaporation rate were in the range of 40 to 60lbs./hr. Pellets produced having a moisture content in the range of 6 to8 percent were firm and hard. Pellets produced having moistureconcentrations of 9 to 10 percent were sufficiently soft to be deformedby finger pressure, but were still sufficiently strong to endure thecascade in the drying section of the pelletizer without deformation orbreaking.

EXAMPLE 4

The pellets produced during the operations described in Examples 2 and 3were placed on a steel grate in the bottom of a square steel enclosuremeasuring 30.5 centimeters on each side to form a bed of pellets 30.5cms. deep. Attached to the side of the enclosure below the grate was aburner of sufficient capacity to develop combustion gas temperatures incombination with secondary air in the range of 150° to 500° C. Theburner and the enclosure below the grate were completely enclosed toforce the flow of heated combustion gases upward through the grate andthrough the bed of pellets. Thermometers were located immediately belowthe grate and also in the uppermost layer of the bed to measure thetemperatures of the gases and the pellets.

The bed, containing pellets measuring about 1.3 cms. in diameter andhaving an initial moisture content in the range of 6 to 8 percent, wascompletely dehydrated when heated to 300° C. in 1 hour. A second bed ofpellets measuring about 1.6 cms. in diameter and having the samemoisture content, was completely dehydrated when heated to 300° C. in1.5 hours. A third bed of pellets in the size range of 1.9 cms. to over2.54 cms. required up to 2.5 hours for complete dehydration by heatingto 300° C. Heating rates more rapid than those indicated above resultedin spalling and explosion of some pellets due to the buildup of vaporpressure within the pellets in excess of atmospheric and also due toexcessive strain due to the high thermal gradients from the surface ofthe pellets inward. Pellets dehydrated and preheated at the aboveheating rates were hard and strong. Anhydrous pellets dropped from aheight of 1.8 to 3 meters onto a hard surface rebounded 10 to 30 percentof the distance dropped without breaking, indicating characteristics ofstrength, hardness, rigidity, and elasticity.

EXAMPLE 5

The specific gravity and crushing strength were determined for layeredspherical glass batch pellets produced by the process of Example 2. Theglass batch pellets had the following composition:

    ______________________________________                                        Component:                                                                              Sand    Dolomite  Feldspar                                                                              Salt Cake                                 ______________________________________                                        % by weight                                                                             70.6    21.2      7.1     1.1                                       ______________________________________                                    

In determining the specific gravity, the average diameters of pellets ofknown weight were determined by direct measurement with calipers of thediameters of eight different areas of each pellet. The specific gravitywas found to be 2.07 grams per cubic centimeter. Crushing strength wasdetermined on a Hounsfield Tensometer and found to be in the range from120 to over 250 lbs.; 250 lbs. being the upper limit of the instrument.In addition, four pellets were analyzed to determine the concentrationof Na₂ O in the core and at the surface of the pellet. The results wereas follows:

    ______________________________________                                                      1    2       3      4     Avg.                                  ______________________________________                                        % Conc. of Na.sub.2 O (core)                                                                  15.11  13.73   14.12                                                                              14.11 14.26                               % Conc. of Na.sub.2 O (surface)                                                               14.29  13.85   13.69                                                                              13.53 13.84                               (analytical accuracy limits: 0.5-1.0%)                                        ______________________________________                                    

The above determinations show that the novel pellets of the presentinvention have excellent crushing strengths and high densities asdesired. The pellets have a homogeneous composition and there is noindication of significant migration of Na₂ O from the core of thepellets to the surface.

EXAMPLE 6

The specific gravity and crushing strength of layered spherical glassbatch pellets, prepared by the process of Example 3 and using thecomposition of Example 3 in which aragonite is the calcium carbonatesource, were determined. The specific gravity was determined by theprocedure used in Example 5 and was found to be 2.13 grams per cubiccentimeter. Pellet crushing strengths in the range of 50 to 90 lbs weredetermined using the Hounsfield Tensometer and the procedure of Example5.

EXAMPLE 7

Layered spherical glass batch pellets were prepared using the glassbatch composition of Example 2 in which calcite was substituted fordolomite as the calcium carbonate source; the composition having thesame weight ratio of components. The specific gravity was measured bythe method of Example 5 and found to be 2.17 grams per cubic centimeter.Pellet crushing strengths were in the range of 120 to over 250 lbs whendetermined by the method of Example 5.

While the preferred structure and process in which the principles of thepresent invention have been incorporated as shown and described above,it is to be understood that the invention is not to be limited to theparticular details thus presented, but in fact, widely different meansmay be employed in the practice of the broader aspects of thisinvention.

Having thus described the invention, what is claimed is:
 1. A continuousprocess for the production of layered pellets for glass production,which comprises the following steps:(a) maintaining a moving bed ofrecycle pellets in the pelletizing zone of a rotary apparatus; (b)feeding sand and particles of a calcium carbonate source into saidpelletizing zone; (c) feeding a Na₂ O source comprised of a solution ofsodium hydroxide into said pelletizing zone, said recycle pellets beingcoated with a layer comprised of said solution of sodium hydroxide, saidsand and said calcium carbonate source to thereby form layered pellets;(d) passing said layered pellets into a heated drying zone to form driedlayered pellets, said dried layered pellets having a residual moisturecontent of from about 4 to about 12 percent by weight; (e) passing saiddried layered pellets from the drying zone to a recycle zone; (f)recycling a portion of said dried layered pellets to said pelletizingzone as said recycle pellets; (g) recovering a portion of said driedlayered pellets from said recycle zone; and (h) feeding said recoveredportion of layered pellets into a static bed dryer for preheating saidpellets through a temperature range of approximately 100° F. to 1400° F.prior to feeding them to a glass furnace while maintaining thetemperature differential between the surface of said pellets and thestatic bed drying gases in the range of 5° F. to 120° F. throughout saiddryer.
 2. The process of claim 1 in which said calcium carbonate sourceis selected from the group consisting of limestone, dolomite, calciumcarbonate, aragonite, calcite, and mixtures thereof.
 3. The process ofclaim 2 in which said layered pellets have as a bonding agentcrystalline sodium carbonate monohydrate.
 4. The process of claim 1 inwhich prior to step (g), a portion of said dried layered pellets ispassed into a classifying zone.
 5. The process of claim 3 in which saidsodium hydroxide solution fed to said pelletizing zone has aconcentration in the range of from about 25 to about 75 percent byweight of NaOH.
 6. The process of claim 5 in which said layered pelletsin said drying zone are heated by passing a stream of gas through saiddrying zone, said layered pellets in said drying zone being maintainedat a temperature in the range from about 20° to about 100° C.
 7. Theprocess of claim 5 in which said calcium carbonate source is admixedwith a minor portion of burnt lime or hydrated lime.
 8. The process ofclaim 5 in which said calcium carbonate source is limestone.
 9. Theprocess of claim 5 in which said calcium carbonate source is dolomite.10. The process of claim 8 or claim 9 in which said Na₂ O source is amixture of sodium carbonate and said sodium hydroxide solution wheresaid sodium carbonate supplies up to about 50 percent of the Na₂ Orequirements.